Camshaft phasers are generally used in valve actuation systems of internal combustion engines to vary the valve opening and closing times, thereby making it possible to improve the fuel consumption figures of the internal combustion engine and the general operating characteristics.
One camshaft phaser design that has proven suitable in practice features a vane-type phaser having a stator and a rotor defining an annular space which is divided by projections and vanes into a plurality of working chambers. The working chambers can be selectively pressurized with a pressure medium which is fed by a pressure medium pump in a pressure medium circuit from a pressure medium reservoir into the working chambers on one side of the vanes of the rotor, and returned to the pressure medium reservoir from the working chambers on the respective other side of the vanes. The working chambers whose volume is thereby increased have a direction of action opposite to that of the working chambers whose volume is decreased. The direction of action accordingly means that pressurizing one of the groups of working chambers with pressure medium causes the rotor to rotate in a corresponding clockwise or counterclockwise direction relative to the stator. The flow of pressure medium, and thus the adjusting movement of the camshaft phaser, is controlled, for example, by a central valve having a complex system of flow passages and control edges and a valve body displaceable within the central valve to close or clear the passage openings as a function of its position.
One problem of such camshaft phasers is that, during a starting phase, they are not yet completely filled with pressure medium, or may even have run empty, so that the rotor may perform uncontrolled movements relative to the stator as a result of the alternating torques exerted by the camshaft. Such uncontrolled movements may lead to increased wear and unwanted noise generation. To avoid this problem, it is known to provide a locking device between the rotor and the stator. When the internal combustion engine is stopped, this locking device locks the rotor relative to the stator in an angular position that is favorable for the starting procedure. In exceptional cases, for example when the engine stalls, it may happen that the locking device does not lock the rotor as intended, and that the camshaft phaser must be operated with the rotor unlocked during the following starting phase. However, since some internal combustion engines have very poor starting performance when the rotor is not locked in the central position, the rotor must then be automatically rotated to the central locking position and locked during the starting phase.
Such automatic rotation and locking of the rotor relative to stator is known, for example, from DE 10 2008 011915 A1 and DE 10 2005 011 916 A1. Both of the locking devices described therein include a plurality of spring-loaded locking pins, which successively lock in locking slots provided in the sealing cover or the stator during a rotation of the rotor. Before the central locking position is reached, the respective locking pins permit rotation of the rotor in a direction toward the central locking position, but inhibit rotation of the rotor in the opposite direction. After the internal combustion engine has warmed up and/or after the camshaft phaser has been completely filled with pressure medium, the locking pins are urged out of the locking slots under the action of the pressure medium, so that the rotor can then be rotated as intended to adjust the angular position of the camshaft relative to the stator.